The invention relates to image acquisition devices such as graphics arts scanners, and more particularly to modeling the spectral behavior of an image acquisition device.
Scanners produce digital output values to represent the spectral reflectance of an input sample. A typical scanner illuminates a reflective target using a light source. The scanners sensors integrate light that reflects off the sample and passes through a set of spectrally selective filters. The integrated products may be modified by electronics and software to obtain digital output values.
The digital values generated by a scanner are dependent upon the filters and illuminant of the particular scanner and so are not device-independent values. To use the results of scanning a sample on a different device, device-independent values are desirable. Device-independent values include the spectral reflectance of the sample or other derived colorimetric values, such as CIE XYZ or CIE L*a*b* values.
In addition, different reflectance spectra can be perceived by the eye as the same color. Similarly, different spectra can produce the same digital values. This effect is called xe2x80x9cmetamerism.xe2x80x9d Metameric samples can produce different colorimetric response values when viewed or scanned under different viewing conditions. As a result, in modeling scanners, many different reflectance spectra can produce the same RGB values. To determine a more accurate estimate of a reflectance spectrum, it is desirable to limit the candidates to avoid metameric matches.
Furthermore, when the number of colorants used to form the gamut from which a sample is obtained is unknown, or when an appropriate physical model for the sample is not known, modeling the reflectance or colorimetric value of the sample becomes difficult. For example, a set of opaque pigmented paints on a substrate may be such a case. Without a limitation on the colorant space for modeling the sample, modeling the digital values of the scanner when scanning such a sample becomes correspondingly difficult.
The invention provides methods and apparatus implementing a technique for modeling spectral characteristics of an image acquisition device. In one implementation, a computer system predicts the spectral reflectance or transmittance of a sample scanned by an image acquisition device, such as a graphic art scanner, by modeling the device. The sample is scanned by a scanner. The computer system searches for media coordinates in a colorant space corresponding to the sample. The media coordinates correspond to an estimated spectrum in a basis spectra model. The basis spectra model is derived by analyzing training color patches of the sample distributed throughout the gamut of the colorant set on the sample media. The estimated spectrum generates estimated digital values through a forward model of the scanner. The estimated digital values are compared to target digital values generated by the scanner to calculate an error value. The computer system repeats this process until a desired stopping criterion or criteria are met. The estimated spectrum corresponding to the final estimated digital values represents the reflectance spectrum of the sample as scanned by the scanner.
In general, in one aspect, the technique includes: (a) estimating media coordinates in a colorant space of a sample scanned by an image acquisition device, where the estimated media coordinates correspond to target digital values produced by the image acquisition device when scanning the sample; (b) converting the estimated media coordinates to an estimated spectrum using a basis spectra model corresponding to the sample; (c) estimating digital values by supplying the estimated spectrum to a forward model which models the image acquisition device; (d) identifying a digital error between the estimated digital values and the target digital values; (e) converting the estimated spectrum to estimated color space values; (f) converting the target digital values to target color space values; (g) identifying a color space error between the estimated color space values and the target color space values; (h) combining the digital error with the color space error to identify a composite error; and (i) if a stopping criterion has not been met, searching the colorant space for media coordinates according to the composite error and repeating steps (b) through (i) until the stopping criterion has been met.
Advantages that may be seen in implementations of the invention include one or more of the following: the final predicted spectrum is a device-independent and viewing-condition-independent approximation of the spectral reflectance of the sample; the model of the scanner can be used to predict the scanner""s output without actually scanning which is useful for experimental purposes; the predicted spectrum is modeled without a supplied media model corresponding to the sample; the number of basis vectors in the basis spectra model is determined automatically without user intervention; constrained minimization searching reduces the potential for metameric matches; and further reduction in metameric matches preferably may be achieved by using a composite error to drive the search.